CN115459619A - Fault-tolerant control method for redundant hot standby MMC - Google Patents
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
- H02M7/53873—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with digital control
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
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- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
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Abstract
The invention relates to a fault-tolerant control method of a redundant hot standby MMC, relating to the technical field of fault-tolerant control of sub-modules of the MMC. According to the invention, an asymmetric phase asymmetric circulation voltage expression is calculated according to a redundant hot standby MMC mathematical model; neglecting the influence of harmonic wave, and calculating an odd-order loop flow expression; then neglecting the influence of the three times of circulating voltage, calculating a fundamental frequency circulating expression, and obtaining the amplitude and the phase angle of the injected fundamental frequency voltage; and finally, injecting the fundamental frequency voltage into the asymmetric phase reference voltage, and switching the sub-modules by adopting a nearest level approximation modulation (NLM) method. The invention is simple and easy to operate, is suitable for asymmetric operation working conditions of a plurality of sub-modules and a plurality of bridge arms, effectively inhibits the circulation fundamental frequency component and harmonic wave in the MMC, obviously reduces the internal loss of the MMC, and improves the output electric energy quality of the MMC.
Description
Technical Field
The invention relates to a fault-tolerant control method of a redundant hot standby MMC, relates to the technical field of safe and reliable operation of the MMC, and particularly relates to the technical field of fault-tolerant control of sub-modules of the MMC.
Background
The Modular Multilevel Converter (MMC) is widely applied to the fields of High Voltage Direct Current (HVDC) and offshore wind power grid connection and the like, due to the characteristics of High modularization degree, good output waveform quality, low device switching frequency and the like. The advantage of the MMC comes from the structure of submodule cascade, however, a large number of cascaded submodules contained in the MMC also constitute a potential fault point, which brings great challenges to the operational reliability of the MMC.
In order to improve the reliability of MMC operation, a hot standby working mode of a redundant submodule is adopted to improve the fault tolerance performance of the MMC submodule. The hot standby working mode of the redundant sub-module puts all sub-modules including the redundant sub-module into operation, and when the sub-module fails, the number of the put-in sub-modules is not reduced by directly bypassing the failed sub-module, so that the MMC can work continuously. However, due to the fact that the number of faults of the sub-modules of the upper bridge arm and the lower bridge arm of each phase is not consistent in actual operation, circulation current of each system is increased due to asymmetric operation, output current is distorted, ripple of capacitor voltage is large, direct current fluctuation is overlarge, and the like.
The method is characterized in that after the fault sub-module is cut off, the circulating current increased due to asymmetric operation can be restrained through redundant fault control, and the method comprises the following solutions of controlling the energy balance of a bridge arm, changing the displacement of a neutral point in the vector direction of a fault phase voltage, introducing a virtual resistor and the like. However, most of the Modulation strategies to be studied are based on Pulse Width Modulation (PWM), and there are few studies on the related art based on Nearest Level Modulation (NLM). The MMC with the NLM has the characteristics of low switching frequency and easiness in implementation, is widely applied to high-voltage occasions with a large number of sub-modules, and the existing open-circuit fault positioning and fault-tolerant control method based on the PWM is not directly applicable.
The current fault-tolerant control method for hot standby MMC comprises the following steps: reducing the sub-module capacitor voltage during normal operation such that both the switch and the capacitor have lower voltage stress and longer service life; all bridge arms are cut off sub-modules, so that the number of all bridge arm input sub-modules is the same, the method can keep symmetrical output characteristics during fault-tolerant operation, but the reliability is reduced; by modifying the sub-module capacitor voltage reference, the frequency shift and phase shift angle of the modulated wave and the carrier wave, the output characteristics of the MMC can still be symmetric without voltage mismatch. However, this fault tolerance procedure is still complex, especially for carrier reconfiguration operations. The asymmetrical fault-tolerant control strategy provided by the invention has the advantages of the fault-tolerant control methods, is simple and easy to operate, and is suitable for asymmetrical operation conditions of multiple bridge arms of multiple sub-modules.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a fault-tolerant control method for a redundant hot standby Modular Multilevel Converter (MMC) based on a recent level modulation (NLM), so as to solve the above problems.
The technical scheme of the invention is as follows: a fault-tolerant control method for a redundant hot standby MMC is characterized in that when the redundant hot standby MMC cuts off the number of upper and lower bridge arm sub-modules of a fault sub-module to be unequal, a fault-tolerant control strategy of injecting fundamental frequency voltage is adopted to restrain fundamental frequency components generated due to the asymmetric structure of the MMC.
The method comprises the following specific steps:
step1: calculating an asymmetric phase asymmetric circulation voltage expression according to a redundant hot standby MMC mathematical model;
step2: neglecting the influence of harmonic waves, and calculating an odd-order circulation flow expression;
step3: neglecting the influence of the third circulation voltage, calculating a fundamental circulation expression to obtain the amplitude and the phase angle of the injected fundamental voltage;
step4: and injecting the fundamental frequency voltage in Step3 into the asymmetric phase reference voltage, and switching the sub-modules by adopting a nearest level approximation modulation (NLM) method.
The Step1 is specifically as follows:
n sub-modules in each bridge arm of the redundant hot standby MMC are necessary sub-modules for maintaining the output level number of the bridge arm, the rest k sub-modules are hot standby sub-modules, and when all the n sub-modules are put into use, the voltage of a bridge arm port is consistent with the voltage of a direct current side. Therefore, from the perspective of the direct-current side voltage, at least k sub-modules of each bridge arm are in an idle state at any moment. The redundancy r refers to the proportional relationship between the number of redundant modules under the hot standby redundancy protection of the MMC and the number of sub-modules necessary for maintaining the normal operation of the system, and specifically comprises the following steps:
when there is excision of n f Redundancy r of failed bridge arm after each failed submodule f Comprises the following steps:
wherein n is f The number of fault submodules.
In order to ensure that the output level number of the whole bridge arm voltage is not changed, the fault redundancy rate r of the sub-module f It is required not to be less than 0,r f When the value is 0, the bridge arm has no redundant submodule, and the asymmetric phase circulating current voltage expression is as follows:
in the formula u cir_asym Representing an asymmetric circulating current component, r fp ，r fn Respectively representing redundancy ratios, I, of upper and lower bridge arms d Representing the direct component, i, in the circulating current h Representing the h-th order loop harmonic component.
The Step2 is specifically as follows:
neglecting the influence of harmonic wave, the phase circulation voltage expression of odd-order frequency components is as follows:
in the formula u cir_odd Representing odd-order components, including fundamental and tripled components.
The Step3 is specifically as follows:
neglecting the influence of the three circulations, obtaining an expression of fundamental frequency circulation:
in the formula u cir_1 Representing the fundamental frequency component.
A large number of cascaded sub-modules are potential fault points of the MMC, and great challenges are brought to the operation reliability of the MMC. Once the fault sub-modules are removed, the MMC works in an asymmetric mode because the number of the bridge arm sub-modules is not equal. The invention discloses a fault-tolerant control method of a redundant hot standby MMC, which introduces odd-order circulating voltage in an asymmetric phase, wherein the fundamental frequency component is large, and fault-tolerant control needs to be realized on the MMC. The asymmetric circulating current causes the distortion of MMC output current, larger capacitor voltage ripple, increased direct current harmonic wave and MMC loss and the like. The conventional circulation controller can only restrain secondary circulation and has no effect on odd circulation, and the invention injects the fundamental frequency voltage based on the redundancy rate in the asymmetric phase by using an injection method.
The beneficial effects of the invention are: the invention is simple and easy to operate, is suitable for asymmetric operation working conditions of a plurality of sub-modules and a plurality of bridge arms, effectively inhibits the circulation fundamental frequency component and harmonic wave in the MMC, obviously reduces the internal loss of the MMC, and improves the output electric energy quality of the MMC.
Drawings
FIG. 1 is a diagram of a half-bridge sub-module MMC structure in an embodiment of the present invention;
FIG. 2 is a flow chart of the basic circulation suppression in an embodiment of the present invention;
FIG. 3 is a graph of MMC symmetric operation circular Fourier analysis in an embodiment of the present invention;
FIG. 4 is a Fourier sub-graph of MMC asymmetric operation without fault-tolerant control in an embodiment of the present invention;
FIG. 5 is a Fourier analysis diagram of fault tolerant control for asymmetric MMC operation in an embodiment of the present invention.
Detailed Description
The invention is further described with reference to the following drawings and detailed description.
Example 1: a fault-tolerant control method for a redundant hot standby half-bridge MMC is disclosed, as shown in figure 1, n sub-modules in each bridge arm are necessary sub-modules for maintaining the output level number of the bridge arm, the rest k sub-modules are hot standby sub-modules, and when all the n sub-modules are put into use, the voltage of a bridge arm port is consistent with the voltage of a direct current side. Therefore, from the perspective of the direct-current side voltage, at least k sub-modules of each bridge arm are in an idle state at any moment. The redundancy r refers to the proportional relation between the number of redundancy modules under the MMC hot standby redundancy protection and the number of sub-modules necessary for maintaining the normal operation of the system:
when a fault submodule is cut off, the redundancy r of a fault bridge arm f Comprises the following steps:
wherein n is f The number of fault submodules. In order to ensure that the output level number of the whole bridge arm voltage is not changed, the fault redundancy rate r of the sub-module f It is required to be not less than 0,r f A value of 0 means that the leg has no redundant submodules.
This example has set up 21 level three-phase contravariant MMC simulation model based on MATLAB/Simulink simulation software platform, and the simulation parameter: the rated capacity S of the system is 200MW, and the peak value u of the phase voltage at the AC side j (j = a, b, c) is 200kV, the voltage of a direct current line is +/-200 kV, the inductance value of a bridge arm is 10mH, and a single bridge arm submoduleThe number of the blocks is 22, and two of the blocks are redundant sub-modules. The sub-module capacitance is 10mF. The modulation mode is the Nearest Level Modulation (NLM), the number of the hot standby redundant sub-modules is 2, and the sub-module capacitor voltage balance control adopts a capacitor voltage-sharing strategy based on complete sequencing.
Further, the method specifically comprises the following steps:
step1: calculating an asymmetric phase asymmetric circulation voltage expression according to a redundant hot standby MMC mathematical model; further, taking phase a as an example, the phase circulation voltage expression is:
wherein u cir_asym Representing an asymmetric circulating current component, r fp ，r fn Respectively represents the redundancy ratio of the upper and lower bridge arms, I d Representing the direct component, i, in the circulating current h Representing the h-th order loop harmonic component.
Step2: neglecting the influence of harmonic wave, the phase circulation voltage expression of odd-order frequency components is as follows:
wherein u cir_odd Representing odd-order components, including fundamental and tripled components.
Step3: neglecting the influence of three circulation currents, obtaining an expression of fundamental circulation currents:
wherein u is cir_1 Representing the fundamental frequency component.
Step4: the fundamental frequency voltage is injected into the asymmetric phase reference voltage, the switching of the sub-modules is performed by adopting a nearest level approximation modulation method (NLM), and the basic loop current suppression flow chart is shown in fig. 2.
The results of the example are shown in FIG. 3: the MMC operates symmetrically, each bridge arm is provided with 22 sub-modules which are put in sequence, at the moment, phase a circulation is shown in figure 3, harmonic in the circulation is mainly 2-order components, and the harmonic distortion rate is lower and is 1.2%. The phase a is asymmetrically operated in the upper bridge arm, one submodule is cut off, the total number of submodules is 21, the other bridge arms are 22, the phase a circulating current is shown in the figure 4, harmonic waves in the circulating current are not only 2-order components, but also odd-order harmonic wave components, a fundamental frequency component is the main component, the amplitude is 261.6A, and the harmonic distortion rate is larger and is 1.55%. As shown in fig. 5, the fault-tolerant control of fundamental frequency voltage injection proposed herein significantly reduces the fundamental frequency component in the circulating current, the amplitude is reduced to 26.9A, and the harmonic distortion rate is also reduced to 1.33%.
While the present invention has been described in detail with reference to the embodiments, the present invention is not limited to the embodiments, and various changes can be made without departing from the spirit of the present invention within the knowledge of those skilled in the art.
Claims (4)
1. A fault-tolerant control method of a redundant hot standby MMC is characterized in that:
step1: calculating an asymmetric phase asymmetric circulation voltage expression according to a redundant hot standby MMC mathematical model;
step2: neglecting the influence of harmonic waves, and calculating an odd-order loop flow expression;
step3: neglecting the influence of the third circulation voltage, calculating a fundamental circulation expression to obtain the amplitude and the phase angle of the injected fundamental voltage;
step4: and injecting the fundamental frequency voltage in Step3 into the asymmetric phase reference voltage, and switching the sub-modules by adopting a nearest level approximation modulation (NLM) method.
2. The method for fault-tolerant control of a redundant hot standby MMC of claim 1, wherein Step1 is specifically:
supposing that n submodules in each bridge arm of the redundant hot standby MMC are necessary submodules for maintaining the output level number of the bridge arm, the redundancy r refers to the proportional relation between the number of the redundant modules under the hot standby redundancy protection of the MMC and the number of the submodules necessary for maintaining the normal operation of a system, and the redundancy r specifically comprises the following steps:
when there is excision of n f Redundancy r of failed bridge arm after each failed submodule f Comprises the following steps:
wherein n is f The number of the fault submodules is;
in order to ensure that the output level number of the whole bridge arm voltage is not changed, the fault redundancy rate r of the sub-module f It is required not to be less than 0,r f When the value is 0, the bridge arm has no redundant submodule, and the asymmetric phase circulating current voltage expression is as follows:
in the formula u cir_asym Representing an asymmetric circulating current component, r fp ，r fn Respectively representing redundancy ratios, I, of upper and lower bridge arms d Representing the direct component, i, in the circulating current h Representing the h-th order loop harmonic component.
3. The method for fault-tolerant control of a redundant hot standby MMC of claim 1, wherein Step2 is specifically:
neglecting the influence of harmonic wave, the phase circulation voltage expression of odd-order frequency components is as follows:
in the formula u cir_odd Representing odd-order components, including fundamental and tripled components.
4. The method for fault-tolerant control of a redundant hot standby MMC of claim 1, wherein Step3 is specifically:
neglecting the influence of the three circulations, obtaining an expression of fundamental frequency circulation:
in the formula u cir_1 Representing the fundamental frequency component.
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